There will be described background art of the present invention with reference to an example of a shovel shown in FIG. 2.
The shovel includes a crawler-type lower traveling body; an upper slewing body 2 disposed on the lower traveling body so as to be able to be slewed around an axis X perpendicular to a ground; a front attachment 3 attached to the upper slewing body 2 to be operated so as to perform excavating operation and so forth; a plurality of hydraulic actuators; and a hydraulic pump that supplies hydraulic oil to the hydraulic actuators. The front attachment 3 includes a boom 4 capable of being raised and lowered; an arm 5 connected to a distal end of the boom 4 rotatably around a laterally horizontal axis; and a bucket 6 connected to a distal end of the arm 5 rotatably around a laterally horizontal axis in the horizontal direction. The plurality of hydraulic actuators include a boom cylinder 7 that raises and lowers the boom 4; an arm cylinder 8 that causes the arm 5 to make rotational movement in a push direction and a retraction direction; a bucket cylinder 9 that causes the bucket 6 to perform excavating and dumping operations; left and right travelling motors that drive the lower traveling body 1 to make it travel; and a slewing motor that slews the upper slewing body 2. Control valves are interposed between the respective hydraulic actuators and the hydraulic pump that is a hydraulic source for the hydraulic actuators. The control valves are operated to control supply and discharge of the hydraulic oil from the hydraulic pump to the respective hydraulic actuators, thereby controlling respective operations of the hydraulic actuators (extension and retraction of each of the hydraulic cylinders, and forward and reverse rotation of the motors).
The hydraulic-cylinder circuit for operating the hydraulic cylinder, included in the hydraulic circuits for operating each of the hydraulic actuators, involves a problem that pressure loss on the return side are increased when the hydraulic cylinder is retracted. This will be described with reference to an example shown in FIG. 3, a circuit for operating the arm cylinder 8.
The circuit includes a hydraulic pump 10, a tank T, a hydraulic-pilot-controlled control valve 11, a not-graphically-shown remote control valve, a head-side pipeline 12, a rod-side pipeline 13, and a return pipeline 14. The control valve 11 is interposed between the arm cylinder 8 and a pair of the hydraulic pump 10 and the tank T, being operated by the remote control valve. The arm cylinder 8 includes a cylinder body, a piston accommodated in a cylinder body; and a rod extending axially from the piston, the piston partitioning the interior of the cylinder body into a head-side chamber (also referred to as a bottom-side chamber) 8a and a rod-side chamber 8b. The head-side pipeline 12 connects the head-side chamber 8a to the control valve 11, whereas the rod-side pipeline 13 connects the rod-side chamber 8b to the control valve 11. The return pipeline 14 connects the control valve 11 to the tank T. The return pipeline 14 is provided with a spring-type check valve and an oil cooler 16, the check valve being a back-pressure valve 15 adapted to generate constant back-pressure. The control valve 11 has a neutral position 11a, an arm-push position (cylinder-retraction position) 11b, and an arm-retraction position (cylinder-extension position) 11c, being switchable over the positions to enable supply and discharge the hydraulic oil to and from the arm cylinder 8 to be controlled, in other words, to enable the extension and retraction operations of the arm cylinder 8 to be controlled.
The return-side pressure loss occurs because the volume of the head-side chamber 8a is greater than that of the rod-side chamber 8b. Specifically, upon the operation of the arm cylinder 8 in the retraction direction, the difference between the volumes of the chambers 8a and 8b causes a large amount of oil to be flowed from the head-side chamber 8a to the return pipeline 14, thereby increasing the pressure loss due to the back-pressure valve 15 and the oil cooler 16 in the return pipeline 14 and thus increasing power loss. In particular, when driven in the push direction, the arm 5 is accelerated by its own weight to thereby especially increasing the pressure loss.
As countermeasures against the above problem, Patent Literature 1 discloses a hydraulic circuit having two return pipelines for leading return oil to a tank when a hydraulic cylinder is retracted. This hydraulic circuit includes a quick return circuit with a relief valve, the quick return circuit connected to one of the return pipelines to allow a part of return oil on the head side, when the hydraulic cylinder is fully retracted, to be flowed to the tank through a path including the quick return circuit. This allows the pressure loss on the return side to be reduced.
This technique, however, requires two high pressure pipes, a relief valve and the like on the return side when the hydraulic cylinder is retracted, thus involving complication of the circuit structure and requiring many parts, which increases cost. In addition, quickly returning the return oil by opening the relief valve raises relief pressure, which weakens the effect of reducing the pressure loss.